One integrated circuit (IC) has output buffers to transmit data signals, for example, onto input/output (I/O) lines of a bus for reception by another IC coupled to the bus. The speed or frequency at which such signals may be reliably transmitted and received on a line depends at least in part, for example, on how well the line is terminated to avoid signal reflections on the line. An IC at an end of a line may use an output buffer having an on-die termination impedance that approximates the characteristic impedance of the line to help avoid signal reflections.
The speed or frequency at which such signals may be reliably transmitted and received on a line also depends at least in part, for example, on the slew rate, that is the rate of voltage change, with which the signals are driven. An IC driving signals at too fast of a slew rate for a given bus frequency may introduce noise on the line and therefore limit the ability of a receiving IC to interpret the signals properly. An IC driving signals at too slow of a slew rate for a given bus frequency may also limit the ability of a receiving IC to interpret signals properly as changing signals may not sufficiently transition from one voltage level to another within the time period in which the receiving IC is to interpret signals.
Because the termination impedance of an output buffer and the slew rate with which the output buffer drives signals can vary due to, for example, variations in process, supply voltage, and/or temperature (PVT variations) for the output buffer, IC's may be designed to drive signals at reduced frequencies to help more reliably transmit and receive signals in the presence of signal reflections and/or with varying slew rates. IC's may also be designed to control termination impedance and slew rates to help maintain them at substantially uniform levels in the presence of PVT variations. One IC controls an output buffer termination impedance and slew rate by controlling resistances affecting termination impedance and slew rate, respectively, relative to the relatively precise resistance of a resistor external to the IC.